1. Field of the Invention
The present invention relates to the field of demodulators and more specifically to radio frequency signal demodulators, the results of which must be exploited by a digital processing unit.
2. Discussion of the Related Art
An example of application of the present invention relates to electromagnetic transponder systems in which a transponder communicates with a contactless read-write terminal.
FIG. 1 very schematically shows, in the form of blocks, the receive demodulation portion of radio frequency signals by an electromagnetic transponder.
After having been received by an antenna and setup by a coupler (not shown), radio frequency signals RF are processed by an analog demodulator 1 (ANALOG DEMOD) having the function of providing a demodulated analog signal AS as well as a sampling clock CK. The sampling clock is generally taken from the transmit carrier. Signals AS and CK are sent onto an interface circuit 2 (INTERF) having the function of providing an output signal O exploitable by a microcontroller 3 (CPU) and other digital circuits. In practice, signal O is provided to a data bus with which CPU 3 communicates. In an application to electromagnetic transponders, a transmission from a terminal to a transponder is performed by using a carrier at 13.56 Megahertz on which are transmitted coded information, generally in amplitude modulation with a non-zero modulation ratio, with a 106-kilobit-per-second flow rate. On the transponder side, the carrier is used to remotely supply circuits of the transponder in the case where the latter has no battery. The same demodulation principle is used in a terminal, except that the clock signal is generally present therein without it being necessary to extract it from the received signal.
FIG. 2 illustrates, in timing diagrams, the principle of a demodulation performed in a circuit such as illustrated in FIG. 1.
The first timing diagram illustrates an example of data D transmitted by signal RF and to be recovered at output O of circuit 2. In this example, it is assumed that the transmission of a bit at state 0 corresponds, within a bit time T, to a low level followed by a high level, while the transmission of a 1 corresponds to the inverse (high level followed by a low level). This is an example only and different types of codings and transmissions could be used. In the example of electromagnetic transponders using a 13.56-Megahertz carrier frequency, time T corresponds, for example, to 106 kilohertz.
At the output of analog demodulator 1, a signal AS (second timing diagram) which roughly follows the shape of signal D is obtained.
The recovered clock signal CK (third timing diagram) corresponds to the carrier signal, that is, to a 13.56-Megahertz frequency. For clarity, the timing diagrams of FIG. 2 are not to scale, especially in the time scale of timing diagrams AS and CK.
The last timing diagram of FIG. 2 shows signal O.
In a conventional circuit 2, signal AS is sampled only once in the middle of a cycle (times t1 and t2). In fact, signal AS is sent to the input of an inverter having its switching threshold TH conditioning the state provided at output O.
A first disadvantage of a conventional demodulation system such as illustrated in FIGS. 1 and 2 is that, if signal AS is strongly disturbed, time t1 or t2 when the signal level is taken into account risks providing an erroneous result.
Another disadvantage is that it is not possible to bring the different times of analysis of signal AS closer to one another, since this would lead to speeds incompatible with the working speed of the CPU having its clock frequency corresponding at most to the frequency of clock CK. Accordingly, the flow rate is limited.
Taking the example of electromagnetic transponder systems based on a 13.56-Megahertz frequency, the limit is in practice 106 kilobits per second to respect the time necessary for a software analysis of the received data.
It would be desirable to be able to increase the transmission speed of such systems. For example, in an application where images are desired to be transmitted (photographs, biometric prints), a flow rate of 106 kilobits per second results in transmission times of several seconds, incompatible with the desired analysis speeds.